Final Fantasy series creator Hironobu Sakaguchi about a year ago was doing a series of “Ask the Gooch” questions on the official Mistwalker YouTube channel and fans had the chance to ask questions which would be answered in his videos. One of the questions came from fan @MattMillsap who asked Sakaguchi what his philosophy was for RPG length and how long it should take to complete the main scenario for the story content. Sakaguchi answered that when he was working on FF1 he wanted to make something that would take about 30-50 hours in length to complete. So if you play for about 2 hours a day it would take an estimated amount of about 20 or so days to finish the game. Basically people that play more on the weekends or take it slow will still not be able to finish the game in just a week. So he feels like a balance between that amount is what an appropriate length for a game should be.

If you are interested in checking out his response here is the video: https://www.youtube.com/watch?v=gtMSThYsfRo&feature=youtu.be&t=8m33s